Pyroprocess is a dry process for treatment of nuclear fuel using chloride molten salts at a high temperature in order to reduce the volume of a spent metal nuclear fuel and reuse the fuel materials. Actinide elements such as uranium (U) and transuranics (TRU) etc. remaining in a molten salt are recovered by electro-refining that uses a solid electrode to isolate pure uranium and by electro-winning that uses a liquid metal electrode to recover uranium and transuranics. For instance, the spent metal nuclear fuel is introduced into an anode in a LiCl—KCl eutectic salt, electric power is applied thereto to melt uranium and transuranic elements, and such uranium and transuranic elements are subjected to electro-deposition on an iron cathode or a liquid cadmium cathode (LCC), thereby recovering uranium and transuranic elements.
Where a large amount of nuclear fuel is subjected to electro-refining and electro-winning, nuclear fission products are accumulated in a molten salt and a process for treatment of waste molten salt is required to eliminate such fission products. A volume of high radiation level waste and an amount of actinide elements for disposal generating from pyroprocessing must be minimized as much as possible, so as to reduce environmental burden and/or problems while improving the economic advantage of the pyroprocess. However, since some actinide elements such as uranium and transuranic elements remain in the molten salt, such actinide elements should be sufficiently removed prior to the treatment process of the waste molten salt.
Conventional methods for recovery of residual actinide elements include, for example: reductive extraction, electrochemical treatment, oxidation and so forth.
In Japan, a number of studies and investigations into development of a countercurrent flow type multi-staged reductive extraction method, which has high practical applicability in order to recover residual actinide elements from waste molten salts, have been conducted at the Central Research Institute of Electric Power Industry (CRIEPI). However, it is difficult to construct a multi-staged reductive extraction apparatus that flows a molten salt and a liquid metal to face each other in three to five stages according to a countercurrent flow way and allows these materials to come into contact with each other in two phases, wherein a difference between specific gravities of both the molten salt and the liquid metal is considerably high and operation of such apparatus is complicated. Moreover, in an aspect of practical utility, when a great amount of waste molten salt generated in a large scaled pyroprocess should be treated, the foregoing multi-staged reductive extraction apparatus requires increased capacity and has difficulties in continuous operation, causing deterioration in processing rate and efficiency.
Argonne national laboratory (ANL) in the United States examined electrochemical recovery method using a Li—Cd anode material and an iron-based solid cathode. However, in such an electrochemical recovery using a solid cathode, disproportionation reaction wherein an electrodeposited metal portion reacts with a trivalent(+3) ion to produce a divalent(+2) ion unavoidably occurs, causing drastic decrease in recovery efficiency.